1. Field of the Invention
This invention relates generally to printing apparatus. It relates more particularly to a calamping mechanism for securing a printing plate to a plate cylinder.
2. Description of the Related Art
In offset lithography, an image is present on a printing plate as a pattern of ink-accepting and ink-repellant surface areas. In a typical sheet-fed offset press, the imaged plate is mounted to a plate cylinder where it is inked and then brought into contact with the compliant surface of a blanket cylinder. The blanket cylinder, in turn, applies the image to paper sheets which are brought into contact with the blanket cylinder by an impression cylinder.
It should be understood at the outset that nowadays a xe2x80x9cprinting platexe2x80x9d may actually be a thin, flexible plastic film or sheet.
Plates can be imaged on-press or, more traditionally on an off-press platesetter. A digitally operated platesetter includes an imaging cylinder to which the plate is initially mounted and which carries the plate past the head of the imaging device. That device transfers the image to the plate. The imaged plate is then removed from the platesetter and transferred to the plate cylinder of the printing press. When mounting an imaged plate to a plate cylinder for a press mount or when mounting a plate to an imaging cylinder for imaging, it is essential that the leading and trailing edges of the plate be secured firmly to the cylinder and that the plate be wrapped tightly around the cylinder. This ensures that there will be no relative movement between the plate and the cylinder when the cylinder is rotated.
A plate cylinder typically includes two plate clamps mounted to the cylinder that extend along its surface. To load a plate onto the cylinder, the leading edge of the plate is secured to the cylinder by one clamp and the plate is wrapped around the surface of the cylinder. The trailing edge of the plate is then secured to the cylinder with the other clamp.
Some printing processes require that two superimposed plates or sheets be mounted independently to the cylinder. For example, in color proofing apparatus, a receptor plate or sheet is secured to the cylinder by a first pair of clamps and successive donor plates or sheets representing color separations are secured to the cylinder over the receptor sheet by a second pair of clamps. After the donor sheet representing each color component has been imaged, that sheet is unclamped from the cylinder and replaced by the donor sheet corresponding to the next color component which must be wrapped around the cylinder and clamped. This process must be repeated three or four times for three or four color printing.
Various devices including hydraulic clamps, vacuum clamps, mechanical and electromechanical clamps have been developed over the years for holding a plate or sheet to a plate cylinder. However, for the most part, these prior devices have tended to be relatively complex. Also, in requiring heavy metal plates as clamps, they take up a relatively large amount of space on the plate cylinder creating a substantial xe2x80x9cvoidxe2x80x9d segment on the cylinder which is the space on the cylinder occupied by the clamps and the space immediately between them. Furthermore, the ancillary mechanisms for operating such clamps, such as air/hydraulic cylinders and lines thereto, take up additional space in the printing apparatus.
The above problems are exacerbated in the case of color proofing apparatus where four such clamps are required to secure the donor and receptor sheets to the cylinder.
The prior clamping devices are also disadvantaged in that the clamping mechanisms are fixed to the plate cylinders such that the mechanisms can only secure to the cylinder a plate or sheet having a specific length. Since the plates are often precut to fit the specific plate cylinder of the printing press, this inability to accommodate different length plates substantially increases the cost of operating and running printing and proofing apparatus.
Accordingly, it is an object of the present invention to provide a speedy and efficient clamping mechanism for clamping a printing plate to a plate cylinder or other support surface.
Another object of the invention is to provide a clamping device for clamping a plate or sheet to a cylinder which takes up a minimum amount of space on the cylinder.
A further object of the invention is to provide a clamping device which allows the associated cylinder to accept plates or sheets of different lengths.
Yet another object is to provide a clamping mechanism which minimizes void space on the cylinder.
Still another object of the invention is to provide a clamping mechanism which does not require bulky ancillary pumps or other apparatus to actuate the plate clamps.
An additional object of the invention is to provide a clamping mechanism of this type which is relatively simple and takes up a minimum amount of space in the imaging, proofing or printing apparatus in which it is installed.
Other objects will, in part, be obvious and will, in part, appear hereinafter. The invention accordingly comprises the features of construction, combination of elements and arrangement of parts which will be exemplified in the following detailed description, and the scope of the invention will be indicated in the claims.
Briefly, our clamping mechanism comprises at least one pair of similar clamping devices for clamping the leading and trailing edges of a plate or sheet preferably wrapped around a rotary cylinder. As alluded to above, in those applications requiring that a second plate or sheet be secured to the cylinder independently of the first, a second similar pair of clamping devices may be required. We will describe the invention as applied to a plate cylinder in an otherwise more or less conventional printing or proofing apparatus. It should be understood however that the invention is useful in other applications in which a thin plate or sheet has to be releasably clamped to a flat or curved surface of one kind or another.
In accordance with the invention, each clamping device is a long thin blade clamp and the cylinder (in whole or in part) and each blade clamp are made of ferromagnetic materials and at least one of the cylinder and clamps is magnetized so that the blade clamp is magnetically attracted to the cylinder with sufficient force to securely clamp the leading or trailing edge of the underlying plate or sheet to the cylinder.
In one embodiment of the invention, each blade clamp is laid down on and retrieved from the cylinder by a separate dispensing mechanism. More particularly, each blade clamp is wound up on a spool which may shuttle along a track extending parallel to the cylinder. The spool is movable along the track between a home position located just beyond one end of the cylinder and an extended position located just beyond the opposite end of the cylinder. The leading end of the blade clamp extending from the spool is terminated by a hook. When the spool is moved from its home position and passes by the adjacent confronting end of the cylinder, the hook engages that end of the cylinder. Resultantly, as the spool travels along the track, the blade clamp is paid out from the spool much like the steel tape of a tape measure, such that the blade clamp is laid down onto the surface of the cylinder (or more specifically onto the printing plate thereon) parallel to the cylinder axis.
When the spool reaches its extended position beyond the opposite end of the cylinder, the trailing end of the blade clamp is released from the spool so that the blade clamp is free to rotate with the cylinder, while the now empty spool remains at its extended position until it is time to unclamp the plate from the cylinder.
In order to unclamp the plate from the cylinder, the rotation of the cylinder is stopped and the cylinder is oriented to position the blade clamp on the cylinder so that its trailing end is aligned with the spool still reposing at its extended position on the track. Then, the spool is moved back along the track toward its home position. As will be described in more detail later, the trailing end of the blade is picked up by the spool and as the spool moves along the track towards home, the blade clamp is rewound onto the spool. This winding up of the blade clamp on the spool peels, rather than pulls, the blade clamp away from the cylinder. Therefore, the motion of the spool along the track is sufficient to overcome the strong magnetic attraction of the blade clamp to the cylinder. By the time the spool reaches the end of the cylinder adjacent to the home position of the spool, substantially the entire length of the blade clamp has been wound up on the spool. Then, as the spool travels the remaining distance to its home position, the hook at the leading end of the blade is released from the end of the cylinder so that the cylinder is again free to rotate independently of the clamping mechanism.
In a second embodiment of our invention, a single blade dispenser or applicator applies all of the blade clamps to, and removes them from, the plate cylinder. As such it is particularly suitable for use in proofing apparatus in which substantially superimposed donor and receptor sheets, drawn from a single roll of sheet material, are clamped to the proofer""s imaging or plate cylinder. In other words, a first pair of blade clamps clamp the leading and trailing ends of the receptor sheet to the plate cylinder and, a second pair of blade clamps clamp the overlying donor sheet to that cylinder.
In this case, the blade dispenser comprises an elongated rotary applicator bar which extends parallel to the cylinder at a clamping station located adjacent to the cylinder. The bar has a polygonal cross-section in that it has a plurality of facets, the number of facets being at least equal to the number blade clamps to be dispensed, i.e. four in this case. In accordance with the invention, a magnet, which may be an electromagnet or a permanent magnet, is present at each facet of the applicator bar which, when operative, is sufficiently strong to attract and hold a blade clamp to the corresponding facet of the applicator bar. As will be described in detail later, the applicator bar may be rotated to selectively position each magnet, and any blade clamp magnetically adhered thereto, opposite the cylindrical surface of the plate cylinder. Also, the dispenser as a whole is movable toward and away from the plate cylinder between an extended position which locates the confronting magnet close to the cylinder surface and a retracted position in which that magnet is spaced away from the cylinder surface. All of the active components of the dispenser, along with the movement of the print cylinder, are controlled by a controller which may also control the other elements of the proofing apparatus.
Since the material for both the donor sheet and the receptor sheet are drawn from a single roll of sheet material, provision is made for separating the donor and receptor sheets from the roll supply after each sheet is wrapped around the plate cylinder and clamped. The dispenser includes a cutting device located at a cutting station adjacent to is the plate cylinder which can be brought into contact with the sheet material on the plate cylinder under the control of the controller at the appropriate times in the blade dispensing cycle as will be described in detail later.
To apply the donor and receptor sheets to the plate cylinder, with the dispenser in its retracted position, four blade clamps are releasably adhered to the applicator bar by the four magnets on that bar, the bar being oriented so that one blade clamp faces the plate cylinder. Then, proofing material from the supply roll is fed to the plate cylinder so that the material is draped over the cylinder with the leading edge of that material positioned between the plate cylinder and the confronting blade clamp on the applicator bar. Next, the dispenser is moved to its extended position so that the operative blade clamp is moved toward the plate cylinder whereby that blade clamp becomes magnetically attracted to the plate cylinder by a force that is stronger than the force holding that blade clamp to the applicator bar (or the electromagnet is deenergized). Then, the dispenser is retracted leaving the blade clamp clamping the proofing material to the plate cylinder.
Next, the controller controls the feeding of additional proofing material from the supply roll and rotates the plate cylinder so that the additional proofing material from the supply roll becomes wrapped around the plate cylinder, while at the same time the controller rotates the applicator bar to position a second blade clamp thereon opposite the plate cylinder. After the plate cylinder has rotated through a selected angle less than 360xc2x0, the plate cylinder is stopped and the dispenser is again moved to its extended position so that the second blade clamp is moved to the proofing material on the plate cylinder and becomes magnetically attracted to the cylinder. The magnet holding the second blade clamp to the applicator bar then releases the blade clamp to the cylinder and the dispenser is retracted leaving the second blade clamp clamping the proofing material to the plate cylinder.
At this point, the controller activates the cutting device which cuts the proofing material just behind the second blade clamp thereby separating the material on the cylinder from the roll supply. That segment of sheet material wrapped around and clamped to the plate cylinder constitutes a receptor sheet for the proofing apparatus and there is a gap between the leading and trailing edges of that sheet, i.e., between the first and second blade clamps.
Similar steps are repeated to apply a donor sheet to the plate cylinder over the receptor sheet. That is, with the dispenser in its retracted position, the applicator bar is rotated to position the third blade clamp adhered thereto opposite the plate cylinder and the plate cylinder is rotated to position the void space thereof opposite the third blade clamp. Also, proofing material is fed from the supply roll so that the leading edge thereof is introduced into the space between the plate cylinder and the third blade clamp. Next, the dispenser is moved to its extended position thereby moving third or operative blade clamp to the void space on the plate cylinder, the third blade clamp being magnetically attracted to that cylinder and released by the applicator bar. The dispenser is then moved to its retracted position and the applicator bar is rotated to position the fourth blade clamp opposite the plate cylinder. At the same time, the plate cylinder is rotated while additional proofing material is fed from the supply roll to the plate cylinder so that additional material overlies the just-applied receptor sheet.
Rotation of the plate cylinder is stopped when the void space thereon is disposed opposite the fourth blade clamp at which point the dispenser is again moved to its extended position so that the fourth blade clamp is released from the applicator bar to the plate cylinder thereby clamping the sheet material to the plate cylinder. Following retraction of the dispenser, the cutting device is activated to sever the sheet material just behind the fourth blade clamp thereby separating the outer layer of sheet material wrapped around the plate cylinder, which constitutes a donor sheet, from the supply roll which roll may then be rotated in the opposite direction to remove the leading edge of the roll supply from the vicinity of the plate cylinder.
With the donor and receptor sheets firmly clamped to the plate cylinder, the proofing apparatus can now be operated to process the sheets in a manner known in the art.
After the proofing operation is completed, the blade dispenser may be operated to remove the blade clamps, in reverse order, from the plate cylinder so as to release only the donor sheet or both sheets from the cylinder. That is, with the dispenser in its retracted position and with one of the magnets on the applicator bar facing the plate cylinder, the plate cylinder is rotated to position the fourth blade clamp opposite the applicator bar and the dispenser is moved to its extended position. If the magnets on the applicator bar are electromagnets, the operative magnet is energized and produces a magnetic field that is stronger than that of the magnetized plate cylinder. Resultantly, the fourth blade clamp becomes attracted to that magnet so that when the dispenser is retracted, the fourth blade clamp is pulled away from the plate cylinder thereby releasing the trailing end of the outer donor sheet. The plate cylinder is then rotated to position the third blade clamp opposite the applicator bar and the applicator bar is rotated to orient a second electromagnet opposite the plate cylinder. Then, the dispenser is extended again and that electromagnet is energized to pick up the third blade clamp from the plate cylinder thereby releasing the leading end of the donor sheet.
If the magnets on the applicator bar are permanent magnets, a stripper mechanism to be described later is inserted under one end of the blade clamp being removed from the cylinder and is moved along under the blade clamp. Resultantly, the blade clamp is progressively moved away from the cylinder a distance such that it has a stronger attraction for the confronting magnet on the applicator bar than for the cylinder. Thus, after the stripper mechanism moves to the opposite end of the blade clamp, the entire blade clamp will have been released by the cylinder to the applicator bar.
Once the clamps holding the donor sheet have been removed as aforesaid, the donor sheet must be removed from the cylinder and replaced by another donor sheet representing another color component which new sheet may then be clamped to the cylinder as described above.
Alternatively, the above procedures may be repeated to remove the second and then the first blade clamp from the plate cylinder thereby unclamping the inner receptor sheet so that both the donor sheet and receptor sheet can be removed from the plate cylinder at the end of a proofing operation.
It should be appreciated from the foregoing that the blade clamp component of our clamping mechanism can be quite thin and only wide enough to provide the necessary magnetic attraction to effectively clamp the leading or trailing edge of the underlying plate to the cylinder. The blade clamp can be laid down anywhere around the circumference of the cylinder simply by properly orienting the cylinder and activating the clamping mechanisms at the proper times.
As will become apparent, the clamping mechanisms utilized to lay down and pick up each blade clamp are relatively simple and occupy a minimum amount of space. Furthermore, they are quite fast and efficient and are relatively easily incorporated into otherwise conventional printing and proofing apparatus.